Diffuse large B cell lymphoma (DLBCL) is the most common form of malignant lymphoma and is diagnosed in over 20,000 patients each year in the US. DLBCL is heterogeneous with respect to morphology, biology, and clinical presentation. By gene expression profiling, at least three molecular subtypes of DLBCL can be distinguished termed Germinal center B cell-like (GC) DLBCL, activated B cell-like (ABC) DLBCL, and primary mediastinal B-cell lymphoma (PMBL). See Alizadeh, A. A., et al, Nature 403(6769), 503-522 (2000). The molecular DLBCL subtypes however differ not only with respect to the expression of thousands of genes, but also have significantly different overall survival rates. GCB DLBCL and PMBL patients respond favorably to conventional treatment. In contrast, ABC DLBCL represents the least curable subtype with 3-year overall survival rates of only 40% following combined therapy with anti-CD20 antibody Rituximab and chemotherapy for the treatment of non-Hodgkin lymphoma (CHOP) regimen or (R-CHOP). In addition, each subtype is characterized by deregulation of distinct oncogenic pathways. ABC DLBCL, for example, is characterized by constitutive nuclear factor-KB (NF-κB) pathway activation predominantly via the CBM (CARD11/BCL10/MALT1) signaling complex, which promotes cell proliferation, differentiation and suppresses apoptosis. See Davis, R. E., et al., J Exp Med, 194(12), 1861-1874 (2001).
Physiologically, activation of the CBM complex in B-cells occurs in response to B-cell receptor (BCR) stimulation. Antigen-binding to the BCR induces receptor oligomerization, which promotes Lyn-mediated phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAM) domains in the B-cell coreceptors CD79A and CD79B. Phosphorylated ITAM domains recruit and activate the spleen tyrosine kinase (SYK) at the plasma membrane, which initiates downstream signaling through Bruton's tyrosine kinase (BTK) and phospholipase C gamma (PLCλ) and ultimately leads to the activation of protein kinase C (PKC). PKCβ is thought to be the predominant PKC isoform mediating BCR-NF-κB activation in B-cells through phosphorylation of the Caspase recruitment domain-containing protein 11 (CARD11, also known as CARMA1). The phosphorylation of the CARD11 linker domain leads to a conformational change that promotes CBM complex assembly. Once activated at the plasma membrane, the CBM complex facilitates the activation of the IKK (I kappa B kinase) complex, which phosphorylates IκBα targeting it for destruction, and thereby allows NF-κB transcription factors to enter the nucleus and drive the expression of NF-κB target genes. While it was long unclear whether NF-κB activation in ABC DLBCL merely reflects the signaling state of the tumor cell of origin, the identification of oncogenic CARD11 mutations in this subtype provided the first evidence for genetic deregulation of this pathway. See Lenz, G., et al, Science 319(5870), 1676-9 (2008). In addition, more recent studies have revealed somatically acquired tumor lesions in several NF-κB pathway regulators, including frequent loss-of function mutations in the negative regulator A20 and genetic abnormalities in CD79A and CD79B. See. e.g., Compagno, M., et al, “Mutations of multiple genes cause deregulation of NF-κB in diffuse large B-cell lymphoma” Nature, 459 (7247), 717-722 (2009); Davis, E. R., et al., “Chronic active B-cell-receptor signalling in diffuse large B-cell lymphoma” Nature, 463, 88-94 (2010)). Thus, it is likely that most, if not all, ABC DLBCL may harbor genetic lesions that constitutively activate NF-κB pathway signaling.
Previous studies showed that ABC DLBCL lines are sensitive to inhibition of CARD11, BCL10, MALT1, or IKKβ, demonstrating a clear dependence on NF-κB pathway signaling. See Ngo, V. N., et al. Nature 441(7089):106-10 (2006). In addition, Davis et al. reported dependency of ABC DLBCL cell lines with wildtype CARD11 on BCR signaling and demonstrated that inhibition of CD79A resulted in cell death. See, Davis et al., Nature, 463, 88-94 (2010). These results contrast a recent study which proposed that ligand-independent ‘tonic’ BCR signaling is a more general feature of B-cell lymphomas that renders these cells dependent on downstream BCR signaling. See Chen, L., et al. Blood 111(4):2230-7 (2008).